1. Field of the Invention
The present invention relates to a modified metal oxide and, more particularly, to a charge-modified metal oxide and toner and developer compositions incorporating same.
2. Background of Related Art
In electrophotography, an image comprising an electrostatic field pattern (also referred to as an electrostatic latent image), usually of nonuniform strength, is formed on an insulative surface of an electrophotographic element. The insulative surface comprises a photoconductive layer and an electrically conductive substrate. The electrostatic latent image may be formed by imagewise photo-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on the insulative surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with a toner composition generally containing a colorant. The toner image is then transferred onto a transfer medium such as paper and fixed thereon by heating and/or pressure. The last step involves cleaning residual toner from the electrophotographic element.
Dry toner compositions used in electrophotography are divided into one-component systems composed of a toner, generally including a binder resin having a colorant dispersed therein, and two-component compositions including a toner and a carrier. Charge control agents are often melt mixed with the toner resin to control the chargeability of the toner during use. In order that toner compositions have process suitability in copying, they are required to be excellent in fluidity, anti-caking properties, fixability, chargeability, cleaning properties, and the like. To improve these properties, particularly fluidity, anti-caking properties, and chargeability, extraparticulate inorganic fine particles are frequently added to toner compositions.
Dispersibility of the inorganic fine particles has a great influence on toner composition characteristics. Particles of poor dispersibility tend to fail to obtain desired effects of improving fluidity, chargeability and anti-caking properties or tend to cause adhesion of toner particles to a photoreceptor due to insufficient cleaning, resulting in image defects.
Inorganic fine particles, such as fumed silicas, may be used to provide good dispersibility, fluidity, and anti-caking properties in toner compositions. Unfortunately, untreated fumed silica normally tribocharges negatively which tends to seriously reduce the chargeability of positively chargeable toner compositions. Because the silica also effects the charge of negatively chargeable toners, it complicates the design and use of negatively chargeable toner compositions. The strong negative charge of fumed silica adversely effects the reproducibility of both positively and negatively chargeable toner compositions by exhibiting great variations of chargeability with environmental changes, resulting in image quality variations.
In an effort to overcome these problems, silica particles have been surface treated with various compounds such as aminosilanes, amino-functional silicone fluids, polyacrylates, quaternary ammonium salts and the like to form positively chargeable silica particles. See, for example, U.S. Pat. No. 4,680,245 to Suematsu et al., U.S. Pat. No. 4,902,570 to Heinemann et al., U.S. Pat. No. 5,376,172 to Tripp et al., and U.S. Pat. No. 5,486,420 to Nishihara et al., for varying surface treating agents for use with colloidal silicas in toner formulations.
Although many charge influencing or controlling additives are known, there remains a need for alternative additives which impart or assist in imparting a positive charge to the toner particles in toner and developer compositions. In addition, there continues to be a need for charge influencing or controlling additives which have improved batch to batch reproducibility and no by-products, thereby reducing variability and manufacturing costs. Furthermore, a need remains for improved additives which provide good dispersibility, fluidity, chargeability and anti-caking properties in toner and developer compositions.